The present invention relates, in general, to packaging semiconductor devices and, more particularly, to packaging semiconductor devices using cap wafers.
In applications such as sensing applications, semiconductor devices such as sensors formed on a device wafer are often hermetically sealed by bonding a cap wafer to the device wafer. The device wafer is then diced into distinct dies. Before bonding the cap wafer to the device wafer, a number of openings are formed in the cap wafer using an etching process. A glass frit is screen printed on the cap wafer. The cap wafer is then aligned to the device wafer so that the solid areas in the cap wafer overlie the semiconductor devices arranged in dies on the device wafer and the openings in the cap wafer overlie the bonding pads on the device wafer. The cap wafer and the device wafer assembly is placed in a furnace to heat the glass frit until the glass frit bonds the cap wafer and the device wafer together, and, this hermetically seals the dies on the device wafer.
As the die density on the device wafer increases, the density of the openings in the cap wafer must increase accordingly. Thus, the cap wafer becomes increasingly fragile, which increases the chance of the cap wafer being broken during the packaging process. An approach to increase the sturdiness of the cap wafer is to increase the thickness of the cap wafer. However, a thick cap wafer requires the openings in the cap wafer having a sloped boundary for accessing the bonding pads in a subsequent wire bonding process. The sloped boundary of the openings in a thick cap wafer consumes a large silicon area, thereby increasing the die size and decreasing the die density on the device wafer. Further, a thick cap wafer sometimes induces stress to the package when bonded to the device wafer and the stress may affect the operation of the semiconductor devices, such as sensors, in the package.
Accordingly, it would be advantageous to have a method for packaging semiconductor devices by bonding a cap wafer to a device wafer. It is desirable for the packaging method to maintain the sturdiness of the cap wafer as the die density on the device wafer increases. It is also desirable for the packaging method to maintain easy access to the wire bonding pads on the device wafer as the die size decreases. It would be of further advantage for the packaging method not to induce stress to the packaged devices.